feat: configurable UI base path via BASE_PATH build arg (#58)

Serves the UI under /ui instead of /. This pairs with the argocd route simplification (argocd-apps#201) where /ui → UI service and everything else → API.

- Vite: `base` set from `BASE_PATH` env var at build time
- React Router: `basename` set from injected `__BASE_PATH__`
- Nginx: location block uses `${BASE_PATH}`, substituted by sed at build
- Dockerfile: `ARG BASE_PATH=/` (default preserves existing behavior)
- Woodpecker: passes `BASE_PATH=/ui` to docker-web build

Tested: assets serve at `/ui/assets/...`, SPA routing works at `/ui/remotes`, etc.
Reviewed-on: #58
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

ui/src/main.tsx

@@ -4,9 +4,13 @@ import { BrowserRouter } from 'react-router-dom';
 import { App } from './App';
 import './index.css';
 
+declare const __BASE_PATH__: string;
+
+const basename = __BASE_PATH__.replace(/\/+$/, '') || '/';
+
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <BrowserRouter>
+    <BrowserRouter basename={basename}>
       <App />
     </BrowserRouter>
   </StrictMode>,